It is well known that metallic materials or products, even with a polished finish, are covered with a thin oxide layer formed by oxidation in air. Known film formation techniques for protecting the surface of a product or for forming a thin film include coating, depositing, anodizing, sputtering, vacuum evaporation, electrodeposition, and so forth. Coating is suitable for obtaining a thick film, but the coating film is non-uniform in thickness and has poor adhesion. While anodizing, sputtering or vacuum evaporation provides a film having a fairly uniform composition with good adhesion, there is obtained only a thin film. Where anodizing is applied to an aluminum substrate, the resulting aluminum oxide layer is not dense. Electrodeposition and anodizing are not suitable for the treatment of powder because an object to be treated must serve as an electrode.
These conventional techniques can easily be carried out in cases where a substrate has a large size. However, they are not applicable to a powdered product without some additional techniques. Even when using additional techniques, it has been difficult to form a film of uniform thickness on the powder surface.
With reference to metal powder, formation of an oxide layer on the surface thereof is not difficult because the surface metal undergoes oxidation on exposure to an oxidizing atmosphere, thereby to form a thin oxide layer spontaneously. However, where the metal is very susceptible to oxidation or where the particle size is small, the spontaneous oxidation process cannot be adopted because the reaction proceeds too rapidly, leading to ignition. If the degree of oxidation is controlled, the resulting oxide layer would be too thin for practical use. While the surface of metal powder may be oxidized with an oxidizing agent in a liquid system, the contact with an oxidizing agent cannot be effected uniformly because of the heterogeneous system so that formation of a metallic oxide layer of uniform thickness has been difficult. If the reaction is controlled so as to form a dense oxide layer, it is difficult to form a thick film. Hence, it has not been easy to form a dense film to a desired film thickness.
It is more difficult to uniformly form an oxide layer of a metal different from the substrate metal powder. Although there is a technique of coating silicon oxide or titanium oxide on metal powder to a very small thickness for the purpose of surface treatment, the technique is accompanied with difficulty in providing a uniform and large thickness. Where depositing and coating techniques, though capable of forming a thick film on a metallic substrate, are applied to metal powder, the metal powder must be kept in a dispersed state. As a result, particles formed solely of the coating substance are likely to be formed, in addition to the desired coated metal powder, only to provide a mixture of the powder of the coating substance and the coated metal powder. No technique is available for coating metal powder with an oxide of a different metal to a large thickness without producing particles solely comprising the metallic oxide.
Various difficulties are also met with in coating a powder of a metallic compound with an oxide of a metal different from that constituting the metallic compound. For example, in the case where a metallic compound is deposited on a powder in a metallic salt aqueous solution, and the deposit is heated to be converted to the corresponding oxide, the aqueous solution is impregnated into the substrate metallic compound. The results is that the deposited metallic compound, such as a metallic oxide, contains a different metallic oxide and that a dense oxide layer cannot be obtained.
It has been proposed to form a silver film on mica, which is a non-metallic object, by calcination and reduction for the purpose of imparting a metallic luster to mica as disclosed in JP-A-1-208324 (the term "JP-A" as used herein means an "unexamined published Japanese patent application). This process, however, involves a heat treatment in a high temperature and therefore cannot be applied to general powdered objects.
Further, KINZOKU HYOMEN GIJUTSU (METAL SURFACE TECHNOLOGY), Vol. 17, No. 8, p. 299 et seq. (1966) reports an electroless plating process for forming a metallic cobalt film on a plate, which comprises immersing a plate object in a cobalt complex salt aqueous solution and reducing the cobalt complex ion. However, these disclosures make no mention of formation of a plurality of layers.
With respect to formation of a metal coating layer on the surface of metal powder or metallic oxide powder, JP-A-3-271376 proposes a process for forming a metallic cobalt coating layer on the surface of a powdered metal, e.g., cobalt, nickel or iron, or a powdered metallic oxide, e.g., ferrite or chromium oxide, by reducing a water-soluble cobalt salt in a wet system. Similarly, JP-A-3-274278 discloses a process for forming a metallic silver coating layer on the surface of a powdered metal, e.g., cobalt, nickel or iron, or a powdered metallic oxide, e.g., ferrite or chromium oxide, by reducing a water-soluble silver salt in a wet system.
JP-A-60-184570 discloses a process for changing a color tone by forming a metallic oxide layer on a metallic oxide powder (mica). In this process, a titanium oxide is prepared by calcination after a titanium hydrate is formed on a surface of the powder in a solution of sulfate. This process, however, is not preferable because all metallic fine particles are dissolved when the particles are put into the solution according to this process.
With the recent advancement in various technological fields, there has been an increasing demand for metal or metallic compound powder having a specific function in addition to the properties essentially possessed by the powder.
For example, conventional magnetic powders, whose color is acceptable for use in conventional black magnetic toners, cannot be used as a material for color magnetic toners. Metal powder having high heat conductivity cannot be used as such as a heat dissipating filler of a sealing compound for semiconductors, because it is required to have electrical insulating properties; metal powder for this use should have a surface layer with sufficient electrical insulating properties. Conventional methods for forming a thin oxide layer on the surface of a powder, which have been regarded as adequate for such purposes as protection of powder and facilitation of mixing of powder with a synthetic resin, etc., no longer meet these new demands. To satisfy these requirements, a powder having a novel structure is urgently required.
For the purpose of developing highly functional metal or metallic compound powders exhibiting specific properties in addition to the properties essentially possessed by the powder, the present inventors have made an effort to provide a metal or metallic oxide layer on the surface of metal or metallic compound powder as a core substrate.
However, it has been difficult to obtain a functional powder of good quality by forming a single coat on a powder substrate. For example, in preparation of white magnetic powder which can be used as a starting material for color magnetic materials, such as a color magnetic toner and a color magnetic ink, a coating layer comprising metallic cobalt or metallic silver may be formed on a powdered magnetic substance, such as metallic iron, ferrite or chromium oxide, according to the disclosure of JP-A-3-271376 or JP-A-3-274278. In this case, however, the coating layer should have a considerably large thickness, and even with a large thickness the resulting coated powder still has insufficient whiteness.